Power Distribution Board for Romi Chassis

Overview

This power distribution board is designed specifically for the Romi chassis as a convenient way to access the chassis’s battery power and pass that on to the rest of the electronics that make up your robot. The board features reverse voltage protection and the patented latching circuit from the Pololu pushbutton power switch, providing a compact, solid-state power switch for your robot that can be controlled with a momentary pushbutton: one push turns on power and another push turns it off.

The board has a small pushbutton already installed and offers convenient points for connecting external pushbutton or tactile switches in parallel. It also offers several alternate pushbutton connection options that result in push-on-only or push-off-only operation, and additional inputs enable further power control options like allowing your robot to turn off its own power. Alternatively, the board can be reconfigured to disable the pushbutton circuit and give control to the small installed slide switch.

The board’s power buses are accessible through a set of 0.1″-spaced pins that are compatible with standard 0.1″ male and 0.1″ female headers, and also through a larger set of holes that are compatible with 3.5mm-pitch terminal blocks (you can combine a 2-pin block and a 3-pin block into a single 5-pin block that spans the three power holes and two ground holes).

Two 1/4″ #2-56 screws and two #2-56 nuts are included for mounting the board to the Romi chassis.

Installing the power distribution board

1. Make sure the Romi chassis battery contacts have been installed in the chassis. The four single contacts should all have solder tabs that protrude through the holes in the top of the chassis. Note: these four battery contacts should have been dropped into the chassis through the openings in the top, which leaves them loosely resting in their slots; this will give you the option to later remove the power distribution board and battery contacts from the chassis as a single piece after soldering.
2. Line up the four slots in the board with the chassis’s four battery contact solder tabs and then screw the board to the chassis using the two included screws and nuts.
3. Solder the four contacts to the board. We recommend not having batteries installed in the chassis while you do this as they can act as heat sinks that make it more difficult to solder. The battery terminal slot in the PCB should be completely filled with solder, as shown in the right picture above.

Using the power distribution board

Power switch circuit

By default, the on-board pushbutton can be used to toggle power: one push turns on power and another turns it off. Alternatively, a separate pushbutton can be connected to the BTNA and BTNB pins and used instead. Multiple pushbuttons can be wired in parallel for multiple control points, and each of the parallel pushbuttons, including the one on the board itself, will be able to turn the switch on or off. The latching circuit performs some button debouncing, but pushbuttons with excessive bouncing (several ms) might not function well with it.

For proper pushbutton operation, the board’s slide switch should be left in its Off position. (Sliding the switch to the On position will cause the board power to latch on, and the switch must be returned to the Off position before the board can be turned off with the pushbutton.)

Alternatively, to disable the pushbutton, you can cut the button jumper labelled Btn Jmp; this transfers control of the board’s power to the on-board slide switch instead. A separate slide or toggle switch can be connected to the GATE pin and used instead.

More advanced control options are available through the button connection pins and four control inputs:

Power distribution

The diagram below shows the layout of the power distribution buses and access points on the board.

• VBAT is connected to the battery contact labelled BAT1+ and provides a direct connection to the battery supply. By default, VBAT is the high side of all six of the chassis’s AA battery cells in series, although this can be reconfigured with the battery jumper (see below).
• VRP provides access to the battery voltage after reverse-voltage protection.
• VSW is the battery voltage after reverse protection and the power switch circuit.
• VREG is not connected to anything by default, but along with the adjacent ground and VSW pins, the VREG pins provide a good place to connect an optional voltage regulator. For example, adding a D24V5F5 step-down regulator would make a regulated 5 V supply available for a microcontroller and other electronics on your chassis.
• BAT2+ provides access to the high side of two AA cells in series. This can be useful if you reconfigure the board to provide two separate battery supplies as described below.

Battery supply configuration

The power distribution board’s default configuration provides battery power, VBAT, from all six of the chassis’s AA cells in series (nominally about 7.2 V with rechargeable batteries or 9 V with alkaline batteries). However, the board’s battery jumper, labelled Bat Jmp, allows you to reconfigure the battery connections to provide two independent supplies: BAT1, with 4 cells in series (nominally 4.8 V rechargeable or 6 V alkaline), and BAT2, with 2 cells in series (nominally 2.4 V rechargeable or 3 V alkaline). Cutting the connection between the BAT1− and BAT2+ pads separates the two sets of batteries, and using solder to bridge the BAT1− and GND pads establishes a common ground between the two new supplies.

Warning: Do not bridge the BAT1− and GND pads without first disconnecting BAT1− from BAT2+. Failing to do so could create a short circuit across the BAT2 batteries.

Simplified schematic diagram

This schematic is also available as a downloadable pdf (147k pdf).

Dimensions

General specifications

Notes:

1

Board only; no screws or nuts.